In the early days of the telephone art, use of the telephone was often confined to communications among users within a local geographic area. As a result and over the years, the economies related to accessing a communications system have lead to telephones in a local area usually being interconnected through a central controller, often called a local central office in the art.
As digital computers came upon the scene, another local community of use was discernible. Hence, a central controller is commonly employed for interconnecting various user terminals. For example, U.S. Pat. No. 3,851,104; entitled "Digital Communications System" and issued Nov. 26, 1974; discloses a time division, multiple access communications system which, under the control of a central terminal, provides communication links among a plurality of user terminals by way of a single communications signal path.
As the digital computer art advanced, parallel advances in the semiconductor art have lead to smaller, relatively inexpensive computers. With the advent of such smaller computers, the technique of central computer control is being abandoned in favor of a distributed control technique. Also, because of the usually bursty nature of digital information, the recent trend has also been toward communications systems having a capability for handling packets of digital information. One such distributed control communications system is disclosed in U.S. Pat. No. 4,063,220; entitled "Multipoint Data Communication System with Collision Detection" and issued Dec. 13, 1977. Indeed, the '220 patent discloses a communications system in which, when a terminal is to start an intended transmission on a communications path, a phased decoder detects the presence of other transmissions on the path and, responsive thereto, delays the intended transmission until no other transmissions are sensed. Once a transmission has started, if an interference (or collision) therewith is detected, a random number generator is used to select an interval of time at the completion of which the next transmission will be attempted.
Collisions being a problem, efforts exist in the art toward providing communication protocols for mitigating the deleterious effects of collisions. For example, one solution, called a slotted contention protocol, is disclosed in U.S. Pat. No. 4,161,786, entitled "Digital Bus Communications System" and issued July 17, 1979, Another solution, called an unslotted contention protocol, is disclosed in U.S. Pat. No. 4,210,780, entitled "Multiple Access Digital Communications System" and issued July 1, 1980. Unfortunately, the efficiency related to known multiple access digital communications system protocols tends to decrease as the digital signal bit rate increases, e.g., in a range of about 50-to-200 megabits per second.
To overcome such problems, the U.S. Pat. No. 4,439,763 by J. O. Limb, entitled "Collision Avoiding System and Protocol For a Multiple Access Digital Communications System", issued Mar. 27, 1984, and herein incorporated by reference, discloses a communications system, stations and protocol which contemplate a communications loop having first and second oppositely directed and interconnected signal paths. At least two stations are coupled to both the first and the second signal paths. A station writes a first signal on the first path and reads a second signal from the second path. In addition, the station reads a third signal from the first path. The third signal is coupled from the first path to the station by a reading arrangement which electrically precedes the arrangement for writing the first signal on the first path. If the station has a packet to transmit, it can overwrite a busy control field of the third signal packet on the first path. Having read the third signal on the first path, a logical interpretation may be made within the station as to whether the first path is busy or not busy. If the first path is not busy, the packet may be written on the first path by overwriting the third sigal thereon. If the first path is busy, the station may overwrite a request control field of the third signal for indicating that the station was unable to transmit the packet. The packets flow around the loop and are monitored on the second path. Eventually all packets will be transmitted and the request control field may be detected at the receive side of a head station for indicating the event that all packets have been transmitted, in which event a new cycle may be started by initializing each station on the loop and by permitting each station to transmit.
Still alternative solutions are being sought to further improve the efficiency of such systems.